Microfracture is commonly performed for cartilage repair but usually results in fibrocartilage. Microfracture augmented by autologous bone marrow concentrate (BMC) was previously shown to yield structurally superior cartilage repairs in an equine model compared with microfracture alone. The current study was performed to test the hypothesis that autologous BMC without concomitant microfracture improves cartilage repair compared with microfracture alone.Autologous sternal bone marrow aspirate (BMA) was concentrated using a commercial system. Cells from BMC were evaluated for chondrogenic potential in vitro and in vivo. Bilateral full-thickness chondral defects (15-mm diameter) were created on the midlateral trochlear ridge in 8 horses. Paired defects were randomly assigned to treatment with BMC without concomitant microfracture, or to microfracture alone. The repairs were evaluated at 1 year by in vitro assessment, arthroscopy, morphological magnetic resonance imaging (MRI), quantitative T2-weighted and ultrashort echo time enhanced T2* (UTE-T2*) MRI mapping, and histological assessment.Culture-expanded but not freshly isolated cells from BMA and BMC underwent cartilage differentiation in vitro. In vivo, cartilage repairs in both groups were fibrous to fibrocartilaginous at 1 year of follow-up, with no differences observed between BMC and microfracture by arthroscopy, T2 and UTE-T2* MRI values, and histological assessment (p > 0.05). Morphological MRI showed subchondral bone changes not observed by arthroscopy and improved overall outcomes for the BMC repairs (p = 0.03). Differences in repair tissue UTE-T2* texture features were observed between the treatment groups (p < 0.05).When BMC was applied directly to critical-sized, full-thickness chondral defects in an equine model, the cartilage repair results were similar to those of microfracture. Our data suggest that, given the few mesenchymal stem cells in minimally manipulated BMC, other mechanisms such as paracrine, anti-inflammatory, or immunomodulatory effects may have been responsible for tissue regeneration in a previous study in which BMC was applied to microfractured repairs. While our conclusions are limited by small numbers, the better MRI outcomes for the BMC repairs may have been related to reduced surgical trauma to the subchondral bone.MRI provides important information on chondral defect subsurface repair organization and subchondral bone structure that is not well assessed by arthroscopy.
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